1. Field of the Invention
This invention relates generally to an angle of arrival measurement system, and more particularly, to an angle of arrival interferometer measurement system for a received signal that has reduced complexity and cost over existing interferometer systems.
2. Discussion of the Related Art
Many situations exist where it is desirable to know the direction from which an RF signal is received. For example, it is desirable to use RF interrogation signals to determine the location of aircraft in flight for traffic separation purposes. An aircraft may transmit an RF beacon signal that is received by other aircraft in its vicinity. The aircraft that receives the beacon signal determines the direction and distance of the signal source to determine the location of the aircraft. The direction of the received signal can be used to map the location of the aircraft on a screen to determine its location relative to other aircraft. Signal direction or angle of arrival (AoA) estimations are also useful in certain military situations. U.S. patent application Ser. No. 09/404,387, filed Sep. 23, 1999, titled "An Interferometer Multi-Arm Antenna System for Accurate Angle of Arrival (AoA) Estimation" discusses providing AoA estimations in this type of environment.
Interferometer antenna systems and AoA estimation algorithms are used to make high precision measurements of the direction of arrival of RF signals. These interferometer systems typically use an antenna base line that includes a linear array of antenna elements separated by known distances. Receivers connected to the antenna elements measure the relative phases of the received RF signals. Digital processing algorithms then calculate the angle of arrival from the phase differences measured between the antenna elements. The spacing between the antenna elements is typically small integers or prime numbers related to half-wavelengths of the received signal to resolve ambiguities that arise as a function of frequency. Ambiguities occur as a result of the periodicity of the sine functions used in the algorithms to calculate the AoA estimates. The resulting ambiguities give several AoA estimation solutions of the direction of the received signal that have to be resolved to identify the actual direction. Guard antenna elements are oriented so that their beam patterns differ in such a way that by measuring the signal power received by them, algorithms can resolve the ambiguities that remain in the direction of arrival determined from the linear array.
FIG. 1 is a block diagram of a known antenna system 10 used to determine the direction of arrival of an RF signal. The antenna system 10 includes an interferometer antenna system 12 and a guard antenna system 14. The interferometer system 12 includes a plurality of antenna elements 16 positioned at predetermined distances from each other, typically at a ratio that is a small integer. Each of the antenna elements 16 is connected to a separate receiver 18 that measures the phase of the received signals. In this example, five interferometer antenna elements 16 are used, however, other numbers of antenna elements could be provided in other applications. In a desirable configuration, the antenna elements 16 are arranged in a linear array, such as along the leading edge of an aircraft wing, but can be configured along any desirable structure.
The guard antenna system 14 includes four antenna elements 20 that are arranged 90.degree. apart from each other to provide directional sensitivity. Because the linear array of antenna elements 16 are only able to determine the direction of the signal within 360.degree., it is necessary to further define the direction of the signal by sensing received power from the elements 20. Each of the antenna elements 20 is connected to a separate receiver 22 that measures the received power. Each of the receivers 18 and 22 are connected to a controller 24 that processes the signals using known AoA algorithms to generate the best estimate of the direction of the received signal.
As is apparent from the discussion of the antenna system 10 above, a separate receiver is used for each separate antenna element 16 and 20. Typically, these types of interferometer antenna systems employ five or six antenna elements to provide the desired signal direction calculation accuracy, without overwhelming cost and space drawbacks. It is possible to provide such an interferometer antenna system having only two antenna elements, but accuracy is sacrificed and the antenna elements have to be located close together, within a half-wavelength of the received signal.
Because receivers are typically complex, and add significant hardware to the overall antenna system, it is desirable to reduce the number of receivers and the complexity of the system. Also, for smaller aircraft, where space constraints are of a significant concern, it is also desirable to reduce the overall system hardware because of space limitations. Further, it is typically desirable in many situations to reduce the overall cost of the system. However, it is desirable to provide these things without reducing the accuracy of the overall system.
What is needed is an interferometer antenna system for determining signal direction that reduces the complexity and cost of the existing systems. It is therefore an object of the present invention to provide such an angle of arrival interferometer system.